Rust binding `ConnectionFuture` requires `Sync` while most Futures do not

[jmayclin]

Problem:

ConnectionFuture requires that the inner future be Sync, but this creates some friction within the ecosystem.

This friction is demonstrated in #4980 , where customers have to spawn a separate tokio task and await on it to use the AWS SDK inside s2n-tls callbacks.

Solution:

We could remove the Sync bound from our futures. There is some good reading here

• https://internals.rust-lang.org/t/what-shall-sync-mean-across-an-await/12020/2
• Future type returned by trait methods should impl Sync dtolnay/async-trait#77 (comment)

Requirements / Acceptance Criteria:

Friction with the AWS SDK is a good touchstone. We should either remove this friction or have a very clearly documented reason we need this friction.